Collision test apparatus

ABSTRACT

A collision test apparatus ( 20 ) comprises input ( 22 ) and output members ( 23 ) formed from incompressible liquids and between which is positioned a sample ( 24 ) whose impact characteristics are to be measured. A compression wave is input to the input member ( 22 ) and propagated through the sample ( 24 ) and into the output member ( 23 ). Suitable detectors ( 33, 34 ) detect the input compression wave, output compression wave and any part of the input compression wave reflected off the sample ( 24 ).

[0001] This invention relates to a collision test apparatus and moreparticularly to such an apparatus for testing impact characteristics ofhuman body parts with structural vehicle parts in collisions.

[0002] It has been proposed to measure impact characteristics of humanbody parts in vehicle collisions using collision test apparatus formedfrom a modification of a so-called Hopkinson bar. A Hopkinson bar is awell known device for measuring the mechanical properties of materialsunder impact loading.

[0003] Such apparatus usually comprises a projectile which impactsagainst an impact bar. The impact of the projectile with the input barcauses a compression incident pulse ε_(I) to be generated in the inputbar which propagates through the input bar. A specimen to be measured ispositioned between the input bar and an output bar and when the incidentpulse ε_(i) reaches the interface between the input bar and thespecimen, a part of the incident pulse is reflected ε_(R) and the otherpart ε_(T) propagates into the specimen where it deforms the specimenand then propagates into the output bar. By positioning strain gauges onthe input bar and output bar respectively, ε_(I) and ε_(R) can bemeasured as can ε_(T). Measurement of ε_(I), ε_(R) and ε_(T) allows thecalculation of mechanical properties (stress σ, strain ε, strain rate{dot over (ε)}) of the specimen by application of the uniaxial elasticwave propagation theory: $\begin{matrix}{{\sigma_{s} = \quad {E\frac{Ao}{As}ɛ_{T}}},} & {{where}\quad \begin{matrix}{E = \quad {{elastic}\quad {modulus}\quad {of}\quad {the}\quad {bar}}} \\{A_{o} = \quad {{cross}\quad {sectional}\quad {area}\quad {of}\quad {the}\quad {bar}}} \\{A_{S} = \quad {{cross}\quad {sectional}\quad {area}\quad {of}\quad {the}}} \\{\quad {specimen}}\end{matrix}} \\{ɛ_{s} = \quad {\frac{2{Co}}{L_{S}}{\int_{ox}^{t}{ɛ_{R}{t}}}}} & {{where}\quad \begin{matrix}{C_{O} = {{elastic}\quad {wave}\quad {speed}\quad {in}\quad {the}\quad {bar}}} \\{L_{S} = {{specimen}\quad {gauge}\quad {length}}} \\{t = {time}}\end{matrix}} \\{{\overset{.}{ɛ}}_{S} = \quad {\frac{2{Co}}{L_{S}}ɛ_{R}}} & \quad\end{matrix}$

[0004] However, the uniaxial elastic wave propagation theory can only becorrectly used if the cross section of the input and output bars ishigher than the cross section of the specimen and the respective barsmust remain elastic as the specimen deforms until fracture. Furthermore,the diameter of the input and output bars must be much smaller than thewavelength of ε_(I). Still further there must not be a large mechanicalimpedance mismatch between input and output bars and the specimen.Therefore the mechanical impedance of the bar Z=ρ_(BAR)*c_(BAR)*A_(BAR)cannot be an order of magnitude higher than the mechanical impedance ofthe specimen z_(SPEC)=ρ_(SPEC)*C_(SPEC)*A_(SPEC)

[0005] where

[0006] ρ=material density

[0007] A=cross sectional area

[0008] C=sound speed

[0009] In fact, if the mechanical impedance mismatch is higher than anorder of magnitude there is nearly total reflection of ε_(I) at theinterface between input bar and specimen and ε_(T) will be so small thatit cannot be recorded with accuracy in the output bar.

[0010] The consequences of mechanical impedance mismatch have hithertolimited practically the application of collision test apparatus formedin this way to the testing of specimens of steel, concrete or mediumdensity (3-4 g/mm²) composite materials. In particular this type ofapparatus has not been capable of being used to measure impactcharacteristics of human soft tissue or other soft materials or bodieswhich stimulate such tissue.

[0011] It is accordingly an object of the present invention to provide acollision test apparatus in which a modified Hopkinson bar system can beused to measure the impact characteristics of soft tissue or other softmaterials and in which the problems associated with impedance mismatchmentioned above are obviated.

[0012] Thus and in accordance with a first aspect of the presentinvention therefore there is provided a collision test apparatuscomprising an input member and output member between which a specimen tobe measured can be positioned, said input member being adapted toreceive an input compression wave and propagate the same to the sampleand said output member being adapted to receive an output compressionwave from said sample and detection means to detect the inputcompression wave, the output compression wave and any part of said inputcompression wave reflected from said sample wherein said input andoutput members comprise incompressible liquids.

[0013] With this arrangement it is possible to provide collision testapparatus in which the impact characteristics of human tissue or othersoft materials can be measured since the mechanical impedance mismatchis reduced to less than an order of magnitude by the use ofincompressible liquids in the input and output members.

[0014] In accordance with a second aspect of the present invention thereis provided a method of testing impact characteristics of a materialcomprising interposing a sample whose characteristics are to be measuredbetween input and output members each comprising incompressible liquids,arranging for an input compression wave to be created in the inputmember and propergated to the sample, detecting the input compressionwave, any output compression wave in the output member and any part ofthe input compression wave reflected from the sample.

[0015] The invention will now be described further by way of exampleonly and with reference to the accompanying drawings in which:

[0016]FIG. 1 shows a schematic representation of a known form ofcollision test apparatus utilising a Hopkinson bar;

[0017]FIG. 2 shows in schematic form a first embodiment of collisiontest apparatus according to the present invention;

[0018]FIG. 3 shows in schematic form a second embodiment of collisiontest apparatus according to the present invention;

[0019]FIG. 4 shows in schematic form a third embodiment of collisiontest apparatus according to the present invention; and

[0020]FIG. 5 shows in schematic form a fourth embodiment of collisiontest apparatus according to the present invention.

[0021] Referring now to the drawings, there is shown in FIG. 1, a knownform of collision test apparatus 10 incorporating a Hopkinson bararrangement.

[0022] The apparatus 10 comprises a gas gun 11 which fires a projectile12 at high speed at an input bar 13. The input bar 13 is linked to anoutput bar 14 and a specimen 16, whose characteristics are to bemeasured, is sandwiched between the input and output bar 13, 14respectively.

[0023] In use, the projectile 12 is fired at high speed at the input bar13 and upon impact creates a compression wave ε_(I) in the input bar 13which travels along the input bar 13 to the interface 17 between theinput bar 13 and the specimen 16. As previously explained, at thisinterface 17, a part of the wave is reflected ε_(R) dependent on thespecimen deformation and on the impedance mismatch between the bar 13and the specimen 16 and the remaining part ε_(T) is transmitted into thespecimen 16 and deforms the specimen 16. The transmitted wave ε_(T) thenexits the specimen 16 and is transmitted into the output bar 14. ε_(I),ε_(R) and ε_(T) are conveniently measured using strain gauges (notshown) mounted on the input and output bars 13, 14 respectively and,from the values measured by the strain gauges, the impactcharacteristics of the specimen can be calculated using the equation setout in the introductory paragraphs of the specification.

[0024] The problem with this arrangement is that it cannot be used tomeasure the properties of a specimen which has a difference inmechanical impedance which is greater than 1 magnitude mismatched withthe mechanical impedance of the input or output bar 13 or 14 such as,for example would be the case with human tissue or other soft materialsince ε_(R) would tend towards 100% and ε_(T) would be almost zero withthe consequence of a low accuracy in the measurement of ε_(T) andtherefore low accuracy in the measurement of the strength of thespecimen.

[0025] A first embodiment of apparatus 20 according to the invention isshown in FIG. 2.

[0026] The apparatus 20 comprises a prestressed loading member 21 whichextends into a liquid input member 22 which is linked to a liquid outputmember 23 by a specimen 24.

[0027] The prestressed loading member 21 comprises a plunger arrangementformed by a plunger member 26 and a plunger plate 27. The plunger member26 is provided with an area of weakness 28. The area of weakness 28 maycomprise a portion of reduced diameter or any other suitableconfiguration which leads to a reduction in the tensile strength in theregion of the area 28. The plunger member 26 extends into one end of theliquid input member 22 through an aperture 29 in an input closure 31such that the plunger plate 27 is provided within the liquid inputmember 22 and plunger member is adapted for connection to a tensioningdevice (not shown).

[0028] The input closure 31 is rigidly connected to ground and acts tosupport the plunger plate 97 in the liquid when the plunger is placedunder tension by the tensioning device.

[0029] The liquid input member 22 comprises a hollow cylindrical member32 containing a non compressible liquid and which is preferably formedfrom steel or aluminium. One end of the input member 22 is closed off bythe input closure 31 through which the plunger arrangement extends. Atleast one piezo electric transducer 33 extends through the wall of thehollow cylindrical member 32, into the interior thereof to measurepressure changes in the liquid and the transducer is disposed at anappropriate distance from the opposite end of the member 22 to the inputclosure 31. At least one strain gauge 34 is also provided on the wall ofthe hollow cylindrical member 32 also at an appropriate distance fromthe opposite end of the input member 22, and the strain gauge 34 acts tomeasure deformation of the circumferential wall of the liquid inputmember 22 due to pressure changes in the liquid therein. The distance ofthe piezo-transducer 33 or strain gauge 34 from the interface betweeninput member 22 and specimen 24 must be sufficient for recordingseparately the incident pulses and the reflected pulses (e.g. 0.5 m).The opposite end of the liquid input 22 member is closed off by a seal36 in the form of a soft membrane.

[0030] The liquid output member 23 is generally similar to the inputmember 22 and an end thereof closest to the input member 23 is alsoclosed off by a seal 38 in the form of a soft membrane and the other endis closed off by an output closure 39. Likewise at least one piezoelectric transducer 41 and at least one strain gauge 42 are providedadjacent to one end at the same distance from the interface between theoutput member 23 and specimen 24 in a manner similar to that describedabove in relation to the liquid input bar 22.

[0031] The incompressible liquid which is provided in the input andoutput members 22, 23 is chosen such that the difference in mechanicalimpedance between the input member 22 and output member 23 with respectto the specimen 24 whose properties are to be measured is defined asfollows:$1 < \frac{A_{Li}\rho_{Li}C_{Li}}{A_{spec}\rho_{spec}C_{spec}} > 10$

[0032] Where

[0033] A_(Li)=cross sectional area of the liquid of input ends outputmember

[0034] P_(Li)=liquid density of input or output member

[0035] C_(Li)=speed of sound in the liquid

[0036] A_(spec), P_(spec), C_(spec) are the same values for the specimen

[0037] Normally it would be simple to achieve this result by arrangementof$\frac{A_{Li}}{A_{spec}} < {10\quad {since}\quad \frac{\rho_{Li}}{\rho_{spec}}\quad {and}\quad \frac{C_{Li}}{C_{spec}}\underset{\_}{\underset{\_}{N}}\quad 1}$

[0038] In use, a soft specimen 24 whose characteristics are to bemeasured is placed between the input and the output member 22 and 23 andis held between the soft seals 36 and 38 which close off ends of theinput and output members 22, 23 respectively. The prestressed loadingmember 21 is tensioned in any suitable manner until the plunger member26 fractures in the region of the area of weakness 28. This causes theelastic potential energy stored in the plunger member 26 to be releasedas a member 26 is driven towards the input member 22 which means thatthe plunger plate 27 moves within the hollow input member 22 and causesthe generation of a longitudinal compression wave pulse P_(I) in theliquid in the input member 22. The compression pulse propagates throughthe input member 22 until it passes through the seal 36 and meets thesoft sample 24. When the compression pulse meets the soft sample 24, dueto the deformation of the specimen 24, a part of the compression wave isreflected P_(R) and another part P_(T) is transmitted into the softsample 24 to deform the sample. The transmitted part P_(T) passesthrough the sample 24 and passes into the output member 23. Furthermore,the use of a incompressible liquid in the input and output members 22,23 means that the pressure changes in the input and output members dueto the generation of the longitudinal compression wave pulses will betransmitted with the same values to the wall of the respective member22, 23. This means that the strain gauges 34, 41 on the input and outputmembers 22, 23 can record the circumferential deformation of the wallε_(CI), ε_(CR), ε_(CT) of the input and output members 22, 23 causedrespectively by the incident pressure pulses in the liquid P_(I), P_(R)and P_(T) as function of time and as a result can calculate the pressurevalues P_(I), P_(R) and P_(T).

[0039] In fact the pressure P inside the liquid in each member isrelated to the circumferential deformation of the wall of the members,which the strain $ɛ_{{circum}^{.}} = \frac{PR}{ED}$

[0040] gauge measures, as follows:

[0041] Where

[0042] P=pressure in liquid

[0043] R=radius of hollow cylindrical member

[0044] E=elastic modulus of hollow cylindrical member material

[0045] D=hollow cylindrical member wall thickness

[0046] Therefore the value of P_(I), P_(R) and P_(T) can be calculatedfrom the following relationships $\begin{matrix}{P_{I} = \quad \frac{ɛ_{CI} \cdot D \cdot E}{R}} & {{where}\quad \begin{matrix}{ɛ_{CI} = \quad {{circumferential}\quad {deformation}}} \\{\quad {{of}\quad {the}\quad {hollow}\quad {cylindrical}}} \\{\quad {{member}\quad {caused}\quad {by}\quad P_{1}}}\end{matrix}} \\{P_{R} = \quad \frac{ɛ_{CR} \cdot D \cdot E}{R}} & {\quad \begin{matrix}{ɛ_{CR} = \quad {{circumferential}\quad {deformation}}} \\{\quad {{of}\quad {the}\quad {hollow}\quad {cylindrical}}} \\{\quad {{member}\quad {provoked}\quad {by}\quad P_{R}}}\end{matrix}} \\{P_{T} = \quad \frac{ɛ_{CT} \cdot D \cdot E}{R}} & {\quad \begin{matrix}{ɛ_{CT} = \quad {{circumferential}\quad {deformation}}} \\{\quad {{of}\quad {the}\quad {hollow}\quad {cylindrical}}} \\{\quad {{member}\quad {provoked}\quad {by}\quad P_{T}}}\end{matrix}}\end{matrix}$

[0047] The values P_(I), P_(R) and P_(T) i.e. the liquid pressure valuescan alternatively or additionally be directly measured by the piezoelectric devices 33, 41 mounted on the input and output members 22, 23respectively.

[0048] Once the values of P_(I), P_(R) and P_(T) as a function of timehave been calculated or directly measured, the mechanical properties ofthe soft sample, such is the stress, strain and strain rate thereincaused by the impact can be calculated using the following equations:$\begin{matrix}{\begin{matrix}{\sigma (t)} \\{ST}\end{matrix} = \frac{A_{LIQUID}{p_{T}(t)}}{A_{ST}}} \\{\begin{matrix}{ɛ(t)} \\{ST}\end{matrix} = {\frac{2}{L_{ST}}{\int_{o}^{t}{{V_{R}(t)}\quad {t}}}}} \\{\begin{matrix}{\overset{.}{ɛ}(t)} \\{ST}\end{matrix} = {\frac{2}{L_{ST}}{V_{R}(t)}}} \\{{V_{R}(t)} = \frac{P_{R}(t)}{\rho_{LIQ}C_{LIQ}}}\end{matrix}$

[0049] Where:

[0050] σ_(ST)=stress in the soft tissue

[0051] ε_(ST)=strain of the soft tissue

[0052] {dot over (ε)}_(ST)=strain rate of the soft tissue

[0053] A_(LIQUID)=cross sectional area of the tissue column

[0054] A_(ST)=cross sectional area of the tissue specimen

[0055] t=time

[0056] C_(LIQ)=wave velocity in the liquid

[0057] L_(ST)=gauge length of the soft tissue specimen

[0058] V_(R)=liquid particle velocity correlated with the reflectedpressure pulse PR.

[0059] The values σ_(st), ε_(st) and {dot over (ε)}_(ST)(t) are valueswhich indicate how the soft sample will react upon an impact.

[0060] It is envisaged that it should be possible to place a human softtissue sample 24 between the input and output members 22, 23 and thecharacteristics of this tissue can be measured directly using theapparatus of the invention. This would not be possible with existingarrangements which employ solid input and output members since themechanical impedance mismatch would result in the impossibility ofrecording P_(I), P_(R) and P_(T).

[0061] As an alternative to measuring the characteristics of the softtissue sample directly, as shown in FIG. 3, an automobile structuralpart 43 can be placed in contact with the sample 24 between the inputand output members 22, 23 and using this arrangement it is possible toquantify the injuries consequence of a collision of this structural parton the tissue. Still further, as shown in FIG. 4, the effect of impactenergy absorption materials 44 on the reduction of the injuriessustained by a human tissue sample 24 can be examined by placing thematerial between the automobile part 43 and the tissue sample 24.

[0062] As a still further alternative, as shown in FIG. 5, it ispossible to, rather than use a prestressed loading bar, activate anautomobile airbag arrangement 46 indirectly in front of the liquid inputmember 22. The activation of the airbag arrangement 46 will result inthe generation of a compression pulse in like manner to that generatedby the use of a prestressed loading bar and accordingly, the incidentcompression P_(I), reflected compression wave P_(R) and transmittedpressure wave P_(T) having measured as described above. Accordingly theeffects of the compression wave pulse on the human tissue sample 24 canbe measured.

[0063] It is of course to be understood that the invention is notintended to be restricted to the details of the above embodiments whichare described by way of example only.

1. A collision test apparatus comprising an input member and an outputmember between which a specimen to be measured can be positioned, saidinput member being adapted to receive an input compression wave andpropagate the same to the sample and said output member being adapted toreceive an output compression wave from said sample and detection meansto detect the input compression wave, the output compression wave andany part of said input compression wave reflected from said samplewherein said input and output members comprising compressible liquids.2. Apparatus according to claim 1, wherein the density of the liquid isless than one order of magnitude difference from that of the sample andthe relationship between the respective densities is defined by thefollowing equation$1 < \frac{\rho_{LIQ}*C_{LIQ}*A_{B_{{AR}_{LIQ}}}}{\rho_{SAMPLE}*C_{SAMPLE}*A_{SAMPLE}} > 10$

where ρ=density, C=elastic wave speed and A=cross-sectional area 3.Apparatus according to claim 2, wherein the liquid is chosen so thatρ_(LIQ)≈P_(SAMPLE) and C_(LIQ)≈C_(SAMPLE) whereby the following equationdefines the relationship $1 < \frac{A_{LIQ}}{A_{SAMPLE}} < 10$


4. Apparatus according to any one of claims 1 to 3, wherein said inputand output members comprise hollow cylindrical members, ends of whichare in contact with said sample and being adapted so as to have adensity substantially equal to that of the sample.
 5. Apparatusaccording to claim 4, wherein the adaptation of the ends comprisesclosure members which close off the end of the respective input andoutput members.
 6. Apparatus according to any one of claims 1 to 5,wherein at least one strain gauge is mounted adjacent each of the inputand output members to measure circumferential deformation ε_(CIRCUMF.)of each said member during testing whereby the pressure in each membercan be calculated according to the following equation:$ɛ_{{CIRCUMF}.} = \frac{PR}{tE}$

where R=member radius, E=elastic modulus of member material and T=memberwall thickness which is directly proportional to pressure pulseamplitude P_(R) or P_(T) in the respective member.
 7. Apparatusaccording to any one of claims 1 to 5, wherein the amplitude of thepressure pulse is measured by a pressure transducer placed in contactwith the input and output member.
 8. Apparatus according of any one ofclaims 1 to 7, wherein the diameter D of the input member and outputmember is at least a factor of 10 smaller than the length of the membersuch that whereby the stress σ_(ST), ε_(ST) and the strain rate {dotover (ε)}_(ST) of the sample can be calculated upon measurements of anincident pressure pulse P_(I) and a reflected pressure pulse P_(R) inthe input member and a transmitted pressure pulse P_(T) in the outputmember according to the following equations: $\begin{matrix}{\begin{matrix}{\sigma (t)} \\{ST}\end{matrix} = \frac{A_{LIQUID}{p_{T}(t)}}{A_{ST}}} \\{\begin{matrix}{ɛ(t)} \\{ST}\end{matrix} = {\frac{2}{L_{ST}}{\int_{o}^{t}{{V_{R}(t)}\quad {t}}}}} \\{\begin{matrix}{\overset{.}{ɛ}(t)} \\{ST}\end{matrix} = {\frac{2}{L_{ST}}{V_{R}(t)}}} \\{{V_{R}(t)} = \frac{P_{R}(t)}{\rho_{LIQ}C_{LIQ}}}\end{matrix}$

Where: σ_(ST)=stress in the soft tissue ε_(ST)=strain of the soft tissue{dot over (ε)}_(ST)=strain rate of the soft tissue A_(LIQUID)=crosssectional area of the tissue column A_(ST)=cross sectional area of thetissue specimen t=time C_(LIQ)=wave velocity in the liquid L_(ST)=gaugelength of the soft tissue specimen V_(R)=liquid particle velocitycorrelated with the reflected pressure pulse PR.
 9. Apparatus accordingto any one of claims 1 to 8, wherein the apparatus further includes animpact mitigation material provided interposed between said sample andsaid input member.
 10. Apparatus according to any one of claims 1 to 9,wherein the input compression wave is generated in the input member byinflation of an inflatable impact mitigation device.
 11. Apparatussubstantially as hereinbefore described with reference to FIG. 2, 3, 4,or 5 of the drawings.
 12. A method of testing impact characteristics ofa material comprising interposing a sample whose characteristics are tobe measured between input and output members each comprisingincompressible liquids, arranging for an input compression wave to becreated in the input member and propergated to the sample, detecting theinput compression wave, any output compression wave in the output memberand any part of the input compression wave reflected from the sample.13. A method according to claim 12 further including the step ofinterposing between said sample and said input member, an impactmitigation material.
 14. A method according of claim 12 or claim 13,wherein the step of arranging for an input compression wave to becreated in said input member is carried out by inflation of aninflatable impact mitigation device.
 15. A method substantially ashereinbefore described with reference to FIG. 2, 3, 4 or 5 of thedrawings.